1. Field of the Invention
This invention relates to a slip control system for a lock-up clutch in a torque converter of an automatic transmission of a vehicle.
2. Description of the Related Art
There has been known an automotive vehicle equipped with a lock-up clutch for establishing a direct engagement of an input member and output member of a torque converter. The lock-up clutch is actuated to be fully engaged ( lock-up condition) under a specific operating condition, in which it is not necessary to amplify the engine torque or to reduce the torque shock due to shift operations in the automatic transmission, so as to increase torque transmitting efficiency of the torque converter, resulting in an improved fuel consumption efficiency. It should, however, be noted that the lock-up clutch, when engaged, produces a problem that an engine vibration and noise are increased, because an engine output shaft is mechanically joined with a transmission gear mechanism.
In view of this, Japanese Patent Public Disclosure 57-33253, laid open to the public in 1982, discloses a transmission control system in which the lock-up clutch is controlled to produce a certain amount of slippage between the input member and the output member of the torque converter so that the engine vibration transmitted to the vehicle body from the transmission is reduced, with the lock-up clutch transmitting a certain amount of engine torque therethrough. In the common slip control, the rotation speeds of the input and output members are detected so that and engaging force of the lock-up clutch is controlled in a manner such that a relative speed or slippage therebetween is converged to a predetermined target value by means of a feedback control ( slip condition ).
Further, in this type of the slip control system, when an acceleration of the vehicle is needed, even under such operating ranges that fully engaging or slip conditions of the lock-up clutch are to be established, the lock-up clutch is fully released to establish a converter condition ( hereinafter referred to as a converter release control). The converter release control is carried out in terms of a feedforward control so that torque amplification effect can be obtained in such an accelerating condition of the vehicle to improve acceleration performance.
It should, however, be noted that there occurs a problem when the converter release control is terminated in response to the termination of the accelerating condition of the vehicle to resume a control to the lock-up condition or slip condition.
The problem is caused in that when the engaging force of the lock-up clutch is abruptly increased in controlling the slippage between the input and output members to the target slippage, the engine speed is undesirably decreased due to a abrupt increase of load acting on the engine output shaft.
In this connection, there is a situation where throttle opening is not returned to a full closed position even when the accelerating condition of the vehicle is substantially terminated. For instance, the driver operates the acceleration pedal to get a slight accelerating condition from an abrupt accelerating condition. In this case, the throttle opening is slightly increased, although the converter release control is terminated. This is because the converter release control is performed only when the throttle valve is opened beyond a predetermined value. As a result, the driver cannot obtain a good feeling in some cases, while driving the vehicle, due to the unexpected engine speed depression.
In order to avoid this problem, it has been suggested that the control to the slip or lock-up condition from the converter release control should not be initiated until a change of the throttle opening is zero or a negative value. However, this suggestion is disadvantageous in that the converter condition is unduly maintained, even where the torque amplification is not needed under the condition. This deteriorates the fuel consumption efficiency.